The pitting on the Cchannel was present when primer was applied to the part. These imperfections or pits caused a stress riser and were the origin of a crack in the Cchannel lightening hole that propagated to the blade's lower skin. The structural integrity of the main rotor blade was compromised. The out-of-track condition of the main rotor blade was exacerbated by the crack that propagated to the lower skin. A torque event inspection of the blade, carried out by company maintenance shortly before the incident, failed to detect any damage or crack on the underside, suggesting that crack propagation may develop rapidly and/or be difficult to detect with the prescribed visual inspection method on that exterior surface of the blade, which is usually painted black. The occurrence blade had accumulated 2981.1 hours TTSN and 24950torque events. The previous investigation, TSB OccurrenceA01P0061, revealed that a failed blade was removed after accumulating a TTSN of 2266.3hours. It was estimated that, that blade had about 32523torque events. No finite torque event number is established for the main rotor blades. The blades essentially reach on-condition status because the manufacturer relies on the inspections to reveal cracks and retire the component. Several main rotor blades were found to have developed cracks at or near STN36.5. The FAA-approved stress analysis for the Model369F Helicopter did not evaluate the stresses applied at this location. MDHI developed the inspection interval based on their evaluation of crack growth analysis. There are no prescribed inspections for identifying lightening hole cracks, or any documented standards to accept material flaws in this part of the main rotor blades, so the crack in the Cchannel rendered the rotor blade not airworthy.5 The blades may not reach their expected life of 3530hoursTIS. In operations where the helicopter is subjected to a high number of torque events, accounting of torque events may be more difficult and imprecise because the helicopter does not automatically register and record the number of times that it transitions from a hover to forward flight, and/or the number of lifts; it relies on the pilot's recollection of the operations. Service bulletinSB369D-201 issued on 15May2001 was annotated as mandatory compliance by MDHI. Under Canadian Aviation Regulations, service letters and bulletins themselves are not mandatory unless mandated by the foreign civil aviation authority or referenced by an airworthiness directive (AD). Hence, MDHI rotor blades are subjected to a fatigue factor that may not be accurately tracked by operators. The manufacturer's method of inspection for the Cchannel consists of running a finger over the lightening holes to check for surface flaws or imperfections, such as burrs or nicks; this may be inadequate because a finger swipe along the length of this channel may not reveal slight imperfections which are best detected with magnification. HTC incorporated an ultrasonic inspection as part of its manufacturing process to detect bonding voids. This inspection may not detect slight sub-surface anomalies or imperfections that are critical to the structural integrity of the main rotor blade and its fatigue life. About a week before the occurrence, the operator secured only one main rotor blade in light wind conditions. Tying down only one blade may exacerbate the bending forces, because the four unsecured blades flutter in the wind and exert leveraged bending forces as they rise and fall. The structural integrity of the main rotor blades may have been compromised. TSB Engineering Branch report LP069/2002, MD Helicopters Inc. study, and SDR databases suggest that the main rotor blades are susceptible to cracking at the first lightening hole, blade STN36.5. Lightening holes in the Cchannel may afford an insignificant weight saving. TSB investigationA01P0061 found that a manufacturing defect at the first lightening hole compromised the structural integrity of the main rotor blade. The following TSB Engineering Branch report was completed: LP 069/2002 - Main Rotor Blades, MD 369 Helicopters (various), C-CHFA, 18 June 2002Analysis The pitting on the Cchannel was present when primer was applied to the part. These imperfections or pits caused a stress riser and were the origin of a crack in the Cchannel lightening hole that propagated to the blade's lower skin. The structural integrity of the main rotor blade was compromised. The out-of-track condition of the main rotor blade was exacerbated by the crack that propagated to the lower skin. A torque event inspection of the blade, carried out by company maintenance shortly before the incident, failed to detect any damage or crack on the underside, suggesting that crack propagation may develop rapidly and/or be difficult to detect with the prescribed visual inspection method on that exterior surface of the blade, which is usually painted black. The occurrence blade had accumulated 2981.1 hours TTSN and 24950torque events. The previous investigation, TSB OccurrenceA01P0061, revealed that a failed blade was removed after accumulating a TTSN of 2266.3hours. It was estimated that, that blade had about 32523torque events. No finite torque event number is established for the main rotor blades. The blades essentially reach on-condition status because the manufacturer relies on the inspections to reveal cracks and retire the component. Several main rotor blades were found to have developed cracks at or near STN36.5. The FAA-approved stress analysis for the Model369F Helicopter did not evaluate the stresses applied at this location. MDHI developed the inspection interval based on their evaluation of crack growth analysis. There are no prescribed inspections for identifying lightening hole cracks, or any documented standards to accept material flaws in this part of the main rotor blades, so the crack in the Cchannel rendered the rotor blade not airworthy.5 The blades may not reach their expected life of 3530hoursTIS. In operations where the helicopter is subjected to a high number of torque events, accounting of torque events may be more difficult and imprecise because the helicopter does not automatically register and record the number of times that it transitions from a hover to forward flight, and/or the number of lifts; it relies on the pilot's recollection of the operations. Service bulletinSB369D-201 issued on 15May2001 was annotated as mandatory compliance by MDHI. Under Canadian Aviation Regulations, service letters and bulletins themselves are not mandatory unless mandated by the foreign civil aviation authority or referenced by an airworthiness directive (AD). Hence, MDHI rotor blades are subjected to a fatigue factor that may not be accurately tracked by operators. The manufacturer's method of inspection for the Cchannel consists of running a finger over the lightening holes to check for surface flaws or imperfections, such as burrs or nicks; this may be inadequate because a finger swipe along the length of this channel may not reveal slight imperfections which are best detected with magnification. HTC incorporated an ultrasonic inspection as part of its manufacturing process to detect bonding voids. This inspection may not detect slight sub-surface anomalies or imperfections that are critical to the structural integrity of the main rotor blade and its fatigue life. About a week before the occurrence, the operator secured only one main rotor blade in light wind conditions. Tying down only one blade may exacerbate the bending forces, because the four unsecured blades flutter in the wind and exert leveraged bending forces as they rise and fall. The structural integrity of the main rotor blades may have been compromised. TSB Engineering Branch report LP069/2002, MD Helicopters Inc. study, and SDR databases suggest that the main rotor blades are susceptible to cracking at the first lightening hole, blade STN36.5. Lightening holes in the Cchannel may afford an insignificant weight saving. TSB investigationA01P0061 found that a manufacturing defect at the first lightening hole compromised the structural integrity of the main rotor blade. The following TSB Engineering Branch report was completed: LP 069/2002 - Main Rotor Blades, MD 369 Helicopters (various), C-CHFA, 18 June 2002 The main rotor blade cracked as a result of metal fatigue, which originated from relatively large pits or imperfections on the Cchannel lightening hole surface. These pits caused a stress riser and were the origin of a crack that propagated to the blade's lower skin. The pitting on the Cchannel was present when primer was applied to the part. The structural integrity of the main rotor blade was compromised by pits before it was surface treated.Findings as to Causes and Contributing Factors The main rotor blade cracked as a result of metal fatigue, which originated from relatively large pits or imperfections on the Cchannel lightening hole surface. These pits caused a stress riser and were the origin of a crack that propagated to the blade's lower skin. The pitting on the Cchannel was present when primer was applied to the part. The structural integrity of the main rotor blade was compromised by pits before it was surface treated. The out-of-track condition of the main rotor blade was exacerbated by the crack in the blade, which propagated to the lower skin. Maintenance interventions to correct for the out-of-track condition were not successful because a crack propagation may develop rapidly and/or be difficult to detect with the prescribed visual inspection. method. Service Bulletin SB369D-201R1 requires that operators track and record in the applicable technical logbook, the number of torque events experienced by the helicopter. Compliance with Service Letters and Bulletins is not mandatory under present Canadian Aviation Regulations. Main rotor blades are subjected to a fatigue factor that may not be accurately tracked by operators because accounting of torque events relies on the pilot's recollection of the operations. The lightening holes on the Cchannel of the main rotor blades may afford an insignificant weight saving and may have slight imperfections that go unnoticed during the quality assurance inspection process; imperfections in the Cchannel may lead to cracking under certain operations and ultimately compromise the structural integrity of the blades. There are no documented standards to accept material flaws in this part of the main rotor blades, so the crack in the Cchannel rendered the rotor blade technically not airworthy. The quality assurance finger swipe inspection method for surface flaws or imperfections at the lightening holes of the Cchannel is inadequate, because these imperfections are best detected with magnification. About a week before the occurrence the operator secured only one main rotor blade in light wind conditions. Tying down only one blade may exacerbate the bending forces, because the four unsecured blades flutter in the wind and exert leveraged bending forces as they rise and fall.Findings as to Risk The out-of-track condition of the main rotor blade was exacerbated by the crack in the blade, which propagated to the lower skin. Maintenance interventions to correct for the out-of-track condition were not successful because a crack propagation may develop rapidly and/or be difficult to detect with the prescribed visual inspection. method. Service Bulletin SB369D-201R1 requires that operators track and record in the applicable technical logbook, the number of torque events experienced by the helicopter. Compliance with Service Letters and Bulletins is not mandatory under present Canadian Aviation Regulations. Main rotor blades are subjected to a fatigue factor that may not be accurately tracked by operators because accounting of torque events relies on the pilot's recollection of the operations. The lightening holes on the Cchannel of the main rotor blades may afford an insignificant weight saving and may have slight imperfections that go unnoticed during the quality assurance inspection process; imperfections in the Cchannel may lead to cracking under certain operations and ultimately compromise the structural integrity of the blades. There are no documented standards to accept material flaws in this part of the main rotor blades, so the crack in the Cchannel rendered the rotor blade technically not airworthy. The quality assurance finger swipe inspection method for surface flaws or imperfections at the lightening holes of the Cchannel is inadequate, because these imperfections are best detected with magnification. About a week before the occurrence the operator secured only one main rotor blade in light wind conditions. Tying down only one blade may exacerbate the bending forces, because the four unsecured blades flutter in the wind and exert leveraged bending forces as they rise and fall. TSB Investigation report A01P0061, released 23May2002, reported that the US Federal Aviation Authority was working on a notice of proposed rule making to expedite an airworthiness directive (AD) to effect mandatory compliance with MD Helicopter Inc. (MDHI) Service BulletinSB369D-201R1. No airworthiness directive has been issued. The FAA is considering an amendment to the field inspection method that will require that the blades be inspected with 10Xmagnification. During investigation A01P0061, Transport Canada noted that compliance with publications recommending the incorporation of modifications, the performance of inspections, or times between overhaul are optional. However, aircraft owners have a duty to be aware of the contents of these publications and to evaluate the need for compliance in light of their own circumstances. Commercial operators should have a formalized process for conducting this evaluation as part of the evaluation program required by Canadian Aviation Regulation 706.15 726.15. Notice of Proposed Amendments (NPAs)2000-249 and2000-250, were approved by the Canadian Aviation Regulatory Advisory Council (CARAC). These NPAs are to amend the CARs, making operators' responsibilities in this matter clear. Transport Canada had stated that they would continue to monitor investigation A01P0061 and consider taking unilateral action through an airworthiness directive should the Federal Aviation Authority not mandate MDHI Mandatory Service BulletinSB369D-201R1. Helifor Industries Ltd. have instituted a policy to require daily main rotor blade torque event inspections on all their 369 series helicopters. Helifor has changed its policy on parking and mooring of the MD500's. They have establishing a minimum separation from the heavy lift helicopters to minimize severe rotor downwash and have changed their tie down equipment. They also performed eddy current inspections on 60main rotor blades with disparate times in service and found 14blades with cracked Cchannel lightening holes. None of these had propagated to the outer skins. Helifor has removed from service any blades that were determined to have cracks. Helifor has performed an eddy current inspection of its blades in service in accordance with Helicopter Technology Company inspection procedure HTCQ-010. This inspection will detect cracks in the Cchannel before they propagate to the skin. Although there is no evidence to establish the propagation rate and that blades have reached retirement with cracks in the Cchannel, Helifor has removed from service any blades that were determined to have cracks. This inspection has been incorporated into our Transport Canada approved Schedule. As well, the requirement for a daily inspection of the blades as per MDHS Service BulletinSB369D-201R1 is in place. As a product improvement, Helicopter Technology Company removed the lightening holes in the Cchannel from new production blades HTC P/N 500P21100-103 and blades sold by MDHI as part number369D21120-503. HTC publishes Mandatory Service Bulletin2100-3R2 on their web site. Canadian Transportation Accident Investigation and Safety Board Act HTC advised the TSB that all blades that come in for repair are Eddy Current inspected.Safety Action TSB Investigation report A01P0061, released 23May2002, reported that the US Federal Aviation Authority was working on a notice of proposed rule making to expedite an airworthiness directive (AD) to effect mandatory compliance with MD Helicopter Inc. (MDHI) Service BulletinSB369D-201R1. No airworthiness directive has been issued. The FAA is considering an amendment to the field inspection method that will require that the blades be inspected with 10Xmagnification. During investigation A01P0061, Transport Canada noted that compliance with publications recommending the incorporation of modifications, the performance of inspections, or times between overhaul are optional. However, aircraft owners have a duty to be aware of the contents of these publications and to evaluate the need for compliance in light of their own circumstances. Commercial operators should have a formalized process for conducting this evaluation as part of the evaluation program required by Canadian Aviation Regulation 706.15 726.15. Notice of Proposed Amendments (NPAs)2000-249 and2000-250, were approved by the Canadian Aviation Regulatory Advisory Council (CARAC). These NPAs are to amend the CARs, making operators' responsibilities in this matter clear. Transport Canada had stated that they would continue to monitor investigation A01P0061 and consider taking unilateral action through an airworthiness directive should the Federal Aviation Authority not mandate MDHI Mandatory Service BulletinSB369D-201R1. Helifor Industries Ltd. have instituted a policy to require daily main rotor blade torque event inspections on all their 369 series helicopters. Helifor has changed its policy on parking and mooring of the MD500's. They have establishing a minimum separation from the heavy lift helicopters to minimize severe rotor downwash and have changed their tie down equipment. They also performed eddy current inspections on 60main rotor blades with disparate times in service and found 14blades with cracked Cchannel lightening holes. None of these had propagated to the outer skins. Helifor has removed from service any blades that were determined to have cracks. Helifor has performed an eddy current inspection of its blades in service in accordance with Helicopter Technology Company inspection procedure HTCQ-010. This inspection will detect cracks in the Cchannel before they propagate to the skin. Although there is no evidence to establish the propagation rate and that blades have reached retirement with cracks in the Cchannel, Helifor has removed from service any blades that were determined to have cracks. This inspection has been incorporated into our Transport Canada approved Schedule. As well, the requirement for a daily inspection of the blades as per MDHS Service BulletinSB369D-201R1 is in place. As a product improvement, Helicopter Technology Company removed the lightening holes in the Cchannel from new production blades HTC P/N 500P21100-103 and blades sold by MDHI as part number369D21120-503. HTC publishes Mandatory Service Bulletin2100-3R2 on their web site. Canadian Transportation Accident Investigation and Safety Board Act HTC advised the TSB that all blades that come in for repair are Eddy Current inspected.